    for a lesson group of the module."
   [moduleCode lessonType lessonGroup]
   (get (get-module-lesson-groups-map moduleCode lessonType) lessonGroup))
+
+(defn- get-set-from-seq-of-maps-with-day-key
+  "Returns a set of days from a sequence of maps, where each map has the `:day`
+   key."
+  [s]
+  (reduce (fn [daySet m]
+            (conj daySet (:day m)))
+          #{}
+          s))
 
 (def ^{:doc     "Width in pixels of a half hour timeslot"
        :private true

          destLessonGroup (data $helper LESSON-GROUP-CHANGE-KEY)

          affectedDaysSet
          (reduce (fn [daySet ttLessonInfo]
                    (conj daySet (:day ttLessonInfo)))
                  #{}
                  Lessons-Created-By-Draggable)]

      (doseq [augTTLessonInfo Lessons-Created-By-Draggable]
        (timetable-remove-lesson! (:day augTTLessonInfo)

                          lessonTypes))

            affectedDaysSet
            (get-set-from-seq-of-maps-with-day-key augTTLessonInfoSeq)]

        ; Remove module from `ModulesSelected`
        (set! ModulesSelected (dissoc ModulesSelected moduleCode))
